This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
With the growing concern on global climate change and the impact of CO2 emissions, emphasis has been placed on CO2 capture from power plants. This concern combined with the implementation of cap-and-trade policies in many countries make reducing CO2 emissions a priority for these and other countries as well as the companies that operate hydrocarbon production systems therein.
Gas turbine combined-cycle power plants are rather efficient and can be operated at relatively low cost when compared to other technologies, such as coal and nuclear. Capturing CO2 from the exhaust of gas turbine combined-cycle plants, however, can be difficult for several reasons. For instance, there is typically a low concentration of CO2 in the exhaust compared to the large volume of gas that must be treated. Also, additional cooling is often required before introducing the exhaust to a CO2 capture system and the exhaust can become saturated with water after cooling, thereby increasing the reboiler duty in the CO2 capture system. Other common factors can include the low pressure and large quantities of oxygen frequently contained in the exhaust. All of these factors result in a high cost of CO2 capture from gas turbine combined-cycle power plants.
At least one approach to lowering CO2 emissions in combined-cycle systems includes stoichiometric combustion and exhaust gas recirculation. In a conventional exhaust gas recirculation system, such as a natural gas combined cycle (NGCC), a recycled component of the exhaust gas is mixed with ambient air and introduced into the compressor section of a gas turbine. Typical CO2 concentrations in the exhaust of a NGCC are around 3%-4%, but can increase above 4% with exhaust recirculation. In operation, conventional NGCC systems require only about 40% of the air intake volume to provide adequate stoichiometric combustion of the fuel, while the remaining 60% of the air volume serves as a diluent to moderate the temperature and cool the exhaust to a temperature suitable for introduction into the succeeding expander. Recirculating a portion of the exhaust gas increases the CO2 concentration in the exhaust, which can subsequently be used as the diluent in the combustion system.
However, due to the molecular weight, specific heat, Mach number effects, etc. of CO2, without significant modifications to either the compressor or the expander sections, standard gas turbines are limited as to the concentration of CO2 that can be tolerated in the compression section of the gas turbine from the exhaust. For example, the limit on CO2 content in the exhaust recirculated to the compression section of a standard gas turbine is about 20 wt % CO2.
Moreover, the typical NGCC system produces low pressure exhaust which requires a fraction of the power produced via expansion of the exhaust in order to extract the CO2 for sequestration or enhanced oil recovery (EOR), thereby reducing the thermal efficiency of the NGCC. Further, the equipment for the CO2 extraction is large and expensive, and several stages of compression are required to take the ambient pressure gas to the pressure required for EOR or sequestration. Such limitations are typical of post-combustion carbon capture from low pressure exhaust associated with the combustion of other fossil fuels, such as coal.
The foregoing discussion of need in the art is intended to be representative rather than exhaustive. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit power generation in combined-cycle power systems.